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     uart — driver for Universal Asynchronous Receiver/Transmitter (UART) devices


     device uart

     device puc
     device uart

     device scc
     device uart

     In /boot/device.hints:

     With flags encoded as:
     0x00010   device is potential system console
     0x00080   use this port for remote kernel debugging
     0x00100   set RX FIFO trigger level to ``low'' (NS8250 only)
     0x00200   set RX FIFO trigger level to ``medium low'' (NS8250 only)
     0x00400   set RX FIFO trigger level to ``medium high'' (default, NS8250 only)
     0x00800   set RX FIFO trigger level to ``high'' (NS8250 only)


     The uart device driver provides support for various classes of UARTs implementing the EIA
     RS-232C (CCITT V.24) serial communications interface.  Each such interface is controlled by
     a separate and independent instance of the uart driver.  The primary support for devices
     that contain multiple serial interfaces or that contain other functionality besides one or
     more serial interfaces is provided by the puc(4), or scc(4) device drivers.  However, the
     serial interfaces of those devices that are managed by the puc(4), or scc(4) driver are each
     independently controlled by the uart driver.  As such, the puc(4), or scc(4) driver provides
     umbrella functionality for the uart driver and hides the complexities that are inherent when
     elementary components are packaged together.

     The uart driver has a modular design to allow it to be used on differing hardware and for
     various purposes.  In the following sections the components are discussed in detail.
     Options are described in the section that covers the component to which each option applies.

     At the heart of the uart driver is the core component.  It contains the bus attachments and
     the low-level interrupt handler.

     The core component and the kernel interfaces talk to the hardware through the hardware
     interface.  This interface serves as an abstraction of the hardware and allows varying UARTs
     to be used for serial communications.

     System devices are UARTs that have a special purpose by way of hardware design or software
     setup.  For example, Sun UltraSparc machines use UARTs as their keyboard interface.  Such an
     UART cannot be used for general purpose communications.  Likewise, when the kernel is
     configured for a serial console, the corresponding UART will in turn be a system device so
     that the kernel can output boot messages early on in the boot process.

     The last but not least of the components is the kernel interface.  This component ultimately
     determines how the UART is made visible to the kernel in particular and to users in general.
     The default kernel interface is the TTY interface.  This allows the UART to be used for
     terminals, modems and serial line IP applications.  System devices, with the notable
     exception of serial consoles, generally have specialized kernel interfaces.


     The uart driver supports the following classes of UARTs:

        NS8250: standard hardware based on the 8250, 16450, 16550, 16650, 16750 or the 16950
        SCC: serial communications controllers supported by the scc(4) device driver.

Pulse Per Second (PPS) Timing Interface

     The uart driver can capture PPS timing information as defined in RFC 2783.  The API,
     accessed via ioctl(2), is available on the tty device.  To use the PPS capture feature with
     ntpd(8), symlink the tty callout device /dev/cuau? to /dev/pps0.

     The hw.uart.pps_mode tunable configures the PPS capture mode for all uart devices; it can be
     set in loader.conf(5).  The dev.uart.0.pps_mode sysctl configures the PPS capture mode for a
     specific uart device; it can be set in loader.conf(5) or sysctl.conf(5).

     The following capture modes are available:
         0x00  Capture disabled.
         0x01  Capture pulses on the CTS line.
         0x02  Capture pulses on the DCD line.

     The following values may be ORed with the capture mode to configure capture processing
         0x10  Invert the pulse (RS-232 logic low = ASSERT, high = CLEAR).
         0x20  Attempt to capture narrow pulses.

     Add the narrow pulse option when the incoming PPS pulse width is small enough to prevent
     reliable capture in normal mode.  In narrow mode the driver uses the hardware's ability to
     latch a line state change; not all hardware has this capability.  The hardware latch
     provides a reliable indication that a pulse occurred, but prevents distinguishing between
     the CLEAR and ASSERT edges of the pulse.  For each detected pulse, the driver synthesizes
     both an ASSERT and a CLEAR event, using the same timestamp for each.  To prevent spurious
     events when the hardware is intermittently able to see both edges of a pulse, the driver
     will not generate a new pair of events within a half second of the prior pair.  Both normal
     and narrow pulse modes work with ntpd(8).

     Add the invert option when the connection to the uart device uses TTL level signals, or when
     the PPS source emits inverted pulses.  RFC 2783 defines an ASSERT event as a higher-voltage
     line level, and a CLEAR event as a lower-voltage line level, in the context of the RS-232
     protocol.  The modem control signals on a TTL-level connection are typically inverted from
     the RS-232 levels.  For example, carrier presence is indicated by a high signal on an RS-232
     DCD line, and by a low signal on a TTL DCD line.  This is due to the use of inverting line
     driver buffers to convert between TTL and RS-232 line levels in most hardware designs.
     Generally speaking, a connection to a DB-9 style connector is an RS-232 level signal at up
     to 12 volts.  A connection to header pins or an edge-connector on an embedded board is
     typically a TTL signal at 3.3 or 5 volts.

Special Devices

     The uart driver also supports an initial-state and a lock-state control device for each of
     the callin and the callout "data" devices.  The termios settings of a data device are copied
     from those of the corresponding initial-state device on first opens and are not inherited
     from previous opens.  Use stty(1) in the normal way on the initial-state devices to program
     initial termios states suitable for your setup.

     The lock termios state acts as flags to disable changing the termios state.  E.g., to lock a
     flag variable such as CRTSCTS, use stty crtscts on the lock-state device.  Speeds and
     special characters may be locked by setting the corresponding value in the lock-state device
     to any nonzero value.  E.g., to lock a speed to 115200, use “stty 115200” on the initial-
     state device and “stty 1” on the lock-state device.

     Correct programs talking to correctly wired external devices work with almost arbitrary
     initial states and almost no locking, but other setups may benefit from changing some of the
     default initial state and locking the state.  In particular, the initial states for non
     (POSIX) standard flags should be set to suit the devices attached and may need to be locked
     to prevent buggy programs from changing them.  E.g., CRTSCTS should be locked on for devices
     that support RTS/CTS handshaking at all times and off for devices that do not support it at
     all.  CLOCAL should be locked on for devices that do not support carrier.  HUPCL may be
     locked off if you do not want to hang up for some reason.  In general, very bad things
     happen if something is locked to the wrong state, and things should not be locked for
     devices that support more than one setting.  The CLOCAL flag on callin ports should be
     locked off for logins to avoid certain security holes, but this needs to be done by getty if
     the callin port is used for anything else.


     The PC Card attachment of this driver is scheduled for removal prior to the release of
     FreeBSD 13.0


     /dev/ttyu?       for callin ports
     /dev/ttyu?.lock  corresponding callin initial-state and lock-state devices

     /dev/cuau?       for callout ports
     /dev/cuau?.lock  corresponding callout initial-state and lock-state devices


     puc(4), scc(4)


     The uart device driver first appeared in FreeBSD 5.2.


     The uart device driver and this manual page were written by Marcel Moolenaar